Updated build docs for OSX

Summary: Ran into some issues building bitcoin-abc on OSX, so I updated the build docs accordingly. I also refactored them slightly to make more sense.

Test Plan: N/A

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D775